PURPOSE: To operate a compressor of an air conditioner in a favorable load condition by a method wherein the compressor main body is normally operated, the compressor main body is operated in a no-load condition by connecting the suction port, the discharge port and the compressing part of the compressor main body to each other when the air conditioner is not operated, and the load on the compressor is gradually increased by interrupting the connection at the time of operating the air conditioner.

CONSTITUTION: The compressor main body 1 is normally operated, and when the air conditioner is not operated, the refrigerant discharged from the discharge port 15 through a discharge passage 16 is returned to the suction port 14 through a bypass passage 17 and a suction passage 13. In the process of compressing the refrigerant which is once sucked into a pump chamber 8 through the suction port 14, the refrigerant is returned to the suction port 14 through a bypass passage 19 and the suction passage 13, so that the compressor main body 1 is operated in the substantially no- load condition. Then, when a fan switch (not shown) is closed so as to operate the air conditioner, valves V1, V2 gradually close the bypass passages 17, 19, respectively, resulting in that the load on the compressor main body 1 is gradually increased. Accordingly, the compressor main body 1 can be brought to the full-load operation without generating any shock on an engine, and the air conditioner can be operated in the rated condition.